Scaling or scale formation generally involves the precipitation and deposition of dense materials on surfaces made of metal and other materials. Scale formation may occur when inorganic mineral salts (such as, for example, calcium carbonates, calcium sulfates, calcium oxalates, and barium sulfates) precipitate from liquids and deposit on the inside surfaces of a system (such as, for example, boilers, evaporators, reactors, cooling water systems, heat exchangers, pipes, filter cloths, reverse osmosis membrane surfaces, oil wells, and desalination evaporators).
Scale formation may cause a number of operational problems, including but not limited to, plugging of equipment, pressure loss, increased utility costs, reduced heat exchange capacity, corrosion, lost production due to downtime, and downgraded products from insufficient feeds. Scaling of equipment may occur in a variety of industries, for example, in paper pulp manufacture, in the chemical or petrochemical industry, in power generation, in water treatment, and in liquor production. For example, in paper pulp manufacturing processes, even though purified water, such as water purified by reverse osmosis, may be used, calcium and barium salts may leach from wood pulp into the processing water. Since sulfuric acid and aluminum sulfate are also used in the paper making processes, sulfate ions may combine with calcium and barium ions to form calcium sulfate and barium sulfate, which generally tend to deposit as scale on the surfaces of the processing equipment. Calcium oxalate scale is also frequently encountered in oxidative bleaching stages, as well as in paper mills that use recycled fiber. Oxalic acid and oxalate ions originate from wood or are formed through oxidative bleaching of carbohydrate and lignin. Oxalic acid is also a metabolic by-product of fungi, typically found in unbleached secondary brown fiber.
With the implementation of elemental chlorine free (ECF) bleaching, many kraft pulp mills encounter even more serious scale-related problems. Four of the most troublesome and frequently encountered scales formed in the ECF bleaching processes are calcium carbonate, calcium oxalate, calcium sulfate, and barium sulfate. Calcium oxalate may often deposit during acidic bleaching stages, and sometimes deposit with calcium sulfate and barium sulfate. At alkaline pH, calcium carbonate and calcium oxalate scales frequently form.
Since pulp is typically the main source of calcium and barium ions, there is no shortage of scale-forming species in an ECF bleach plant, even when purified water is used. The presence of scale-forming species, the abrupt pH changes, temperature shocks, intense mechanical and hydrodynamic shear forces, sudden pressure drops, non-uniform substrate surfaces can all contribute to scale formation, among other reasons.
As another example, in the petroleum industry scale deposition costs millions of dollars each year and is generally thought to be the leading cause in production decline worldwide. Scale can be deposited in various equipment along various water paths, including but not limited to piping, injectors, reservoirs, and surface equipment. Scale formation at oil-producing wells may eventually result in lower oil yields and in well failure. Scale found in oil fields may form by direct precipitation from naturally-occurring water in reservoir rocks, or as a result of produced water becoming oversaturated with scale-forming species when two incompatible waters combine. When an oil or gas well produces water, or water injection is used to enhance recovery, scale may also form. Scale is recognized as one of the top production problems in regions that are prone to scale, such as the North Sea, the US, and Canada.
Natural water in oilfields may contain dissolved substances acquired through contact with mineral phases in the natural environment. Deep subsurface water may be enriched in soluble ions through alteration and dissolution of minerals. The water in sandstone reservoirs or geological formation water that have contact with brine sources may contain abundant scale-forming ions, including but not limited to Ca2+, Mg2+, Ba2+, Sr2+, Na+, K+, CO32−, SO42−, and Cl−. Sea water is also generally rich in scale-forming ions, such as ions that are by-product of marine life and water evaporation. In off-shore oil production, the formation of sulfate scales may occur when sea water, which may be rich with SO42−, and formation water containing high concentrations of barium and calcium are mixed.
Oilfield scale may form when the state of any natural fluid is perturbed such that the solubility limit for one or more components is exceeded. Temperature and/or pressure changes, pH shift, out-gassing, and/or the contact of incompatible water may cause the water to become oversaturated with scale-prone species and lead to the formation of scales.
Barium and strontium sulfate scales may, for example, be particularly troublesome when sulfate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. Barium and strontium sulfate generally form very hard, very insoluble scales that can be difficult to prevent by conventional chemical-based scale inhibition techniques. In some instances, this can be particularly troublesome, as barium and strontium sulfates can be co-precipitated with radium sulfate, making the scale mildly radioactive. Dissolution of sulfate scales in piping is generally difficult, possibly requiring one or more of high pH, long contact times, heat, high pressure, and high velocity circulation.
Barium sulfate, or other inorganic supersaturated salts, may precipitate onto the formation to form a scale, thereby clogging the formation and restricting the recovery of oil from the reservoir. The insoluble salts may also precipitate onto production tubing surfaces and associated extraction equipment that may, for instance, limit productivity, limit production efficiency, and compromise safety. Certain oil-containing formation waters are known to contain high barium concentrations of 400 ppm and higher. Since barium sulfate forms a particularly insoluble salt, the solubility of which declines rapidly with temperature, it may be difficult to inhibit scale formation and to prevent plugging of the oil formation and topside processes and safety equipment.
As a further example, mineral scale formation is a costly design and operating problem in seawater desalination processes. The problem is common to both reverse osmosis (RO) and multiple stage flash (MSF) evaporative processes. For technical and economic reasons, in MSF processes maximum brine temperature and reject brine concentration should generally be as high as possible; however, optimum high temperature distillation of seawater cannot generally be realized without effective control of scale formation, especially calcium scalants. The high temperature required for high efficiency in MSF evaporators may cause formation of hard tenacious scale from calcium sulfate anhydrite. Mineral scale, such as calcium carbonate, magnesium hydroxide, barium sulfate, calcium sulfate, corrosion products such as iron oxide and other deposits such as silica manganese oxide etc. are the most troublesome types of foulants. Although alkaline scale may be generally controlled by acidifying the water, increased corrosion to the system at low pH may create another problem. As for reverse osmosis processes, one of the principal causes of system shutdown and premature membrane replacement is membrane fouling, whereby scale deposits impede the flow of fluid and increase the pressure differential across a system element.
As a further example, scaling is also common problem in power plant cooling and heat transfer systems. Poor condenser performance may be one of the single largest causes of energy loss during power generation. Heat exchanger tube scaling and fouling may also cause turbine backpressure and decrease power plant efficiencies. Scaling may further reduce the performance of a heat exchanger itself, since the thermal conductivity of calcium carbonate is about 25 times lower than that of the steel from which industrial scale heat exchanger tubes are often constructed.
Scaling may also be an especially difficult problem in recirculating power plant systems, wherein high cycles of concentration can occur. For example, scaling may cause problems in cooling towers, as the film fill can be susceptible to various types of deposition. Because of evaporation (for example, 1.8% of the circulation per 10° C. of cooling) in the tower, minerals and organic substances in the recirculating water may concentrate to such a level that scaling can occur.
Cooling towers are used to remove heat from a wide range of industrial processes. Cooling towers traditionally require the use of a chemical treatment maintenance program to prevent scaling, corrosion, and biological fouling. In an exemplary traditional cooling tower set-up, re-circulating water is pumped from a basin into the heat exchange process (such as an air conditioning or refrigeration process). The effluent warm water from the heat exchange process is then pumped to the cooling tower. In the cooling tower, the water is sprayed onto wet decking (internal fill material designed to increase the surface area of the water), thus maximizing evaporation. Air is blown through the tower in a crossflow, counterflow, or parallel direction to the water flow. The water is cooled mainly through evaporative cooling as well as through some amount of sensible heat exchange (heat transfer from water to air). The cooled water is then pumped back to the basin, where the cycle continues. Make-up water is added to the basin to compensate for evaporation and water discharged as blowdown. Because of the high evaporation loss, the water recycling in a cooling tower system may be concentrated quickly, and thus perhaps more prone to scale formation. The concentrated water may be discharged as blowdown, and fresh water added to make up for the volume. Such operation generally leads to extraordinarily high volume of water usage and waste water discharge.
As a further example, scaling is also common in chemical industries, for example where the process used involves a large amount of water or involves the precipitation of solids. In particular, phosphoric acid producers generally have to contend with scale formation in all piping and equipment, which may be caused by the phosphoric acid and gypsum produced. Heat exchangers may also have scaling problems where hard water is used or where the recycling water becomes over saturated.
Scale deposition may cost billions of dollars each year. In Britain alone, the formation of scales—and the loss in efficiency and required maintenance (descaling)—in industrial process plants where water is heated or used as a coolant is estimated to cost about £1 billion per year. Such costs may be attributed to cleaning (e.g., descaling) or the poor thermal conductivity of scaled surfaces. The cost associated with scaling is generally thought to be much higher in the United States.
Scale formation can be divided into several steps. Concentrations of cationic and anionic ions, such as Ca2+, Mg2+, Ba2+, CO32−, SO42−, C2O42−, among others, may increase to concentrations that exceed solubility limits and combine to form ion pairs or salt molecules. Those salt molecules or ion pairs may then form microaggregates, which may further grow into nucleation centers for crystallization. Microcrystals may then form from the nucleation centers to become seeds, which may grow and agglomerate, and may precipitate and adhere to surfaces to grow into large crystals. After adhering to surfaces, these crystals may continue to grow and eventually form an adherent layer of scale on a surface. The crystal scale layer may continue to grow and build up, ultimately forming a scale deposit.
Various chemical anti-scalants, such as chelants, phosphates or phosphonates (organophosphates), polycarbonates, and components of polymers, have been developed to inhibit or reduce the formation of inorganic scales. These chemical anti-scalants typically work by one of the following mechanisms: precipitation threshold inhibition, dispersion, and crystal distortion/modification.
Precipitation threshold inhibition may be achieved, for example, by combining a chelant with scale-forming cations to form a stable complex that interrupts ion-pair formation of scale molecules and inhibits the nucleation of scale crystals. Another type of precipitation threshold inhibitor are chemicals that have multiple attachment sites and can inhibit the growth of microcrystals after nucleation by occupying the active growth sites of microcrystals and blocking access to scale-forming ions. Additional examples that can be classified as precipitation threshold inhibition are ion exchange softening and acidification. Ion exchange softening involves exchanging calcium and magnesium ions with sodium, and acidification involves removing one of the reactants necessary for carbonate precipitation through acid addiction.
For dispersion, anionic chemical dispersants modify the surface charges of scaling crystals such that the crystals are dispersed in solution and do not adhere or adsorb to each other to form scales. Anionic dispersants generally modify scaling crystals by adsorbing onto the surface of growing crystals, thereby increasing the anionic charge of the growing crystals and increasing the electrostatic charge repulsion between the crystals. A high anionic surface charge may increase the activation energy barrier to crystal agglomeration, which in turn produces a more stable dispersion of the colloidal microcrystals. Therefore, chemical dispersants may effectively prevent scaling by retarding crystal agglomeration. Anionic polymers containing carboxylic acid groups may be efficient chemical dispersants.
For crystal distortion/modification, some chemicals may be used to alter the crystal forms or shapes of growing crystals such that crystal adsorption or agglomeration is retarded and the deposit of scales to surfaces is reduced. These anti-scalant chemicals may selectively adsorb onto growing crystals, altering their surface properties and disrupting the lock-and-key fit between precipitating scaling species and the growing crystals. Modifying the crystal shape and reducing the numbers of contact surfaces not only may slow the rate of crystal growth, but may make it difficult for the crystals to form hard, tenacious deposits. The modified crystals may then be swept away from surfaces by process flows. Chemical anti-scalants, which act primarily as either threshold inhibitors or dispersants, may also function as crystal modifiers since they adsorb onto the crystal surfaces.
Chemical anti-scalants based on those three mechanisms discussed above, however, are not always effective due to the complexity of scale formation. For example, precipitation threshold inhibitors that use a chelant have the disadvantage of reacting on a stoichiometric ratio (i.e., one molecule of chelant reacts with, for example, one calcium ion), which may impose very high costs if a large volume of liquid needs to be treated. In addition, chemical anti-scalants may not work due to dissociation under high process temperature or pressure, or due to interference caused by impurities from the process water. Furthermore, besides high cost and low efficiency, chemical anti-scalants generally pose safety and environmental concerns.
U.S. Pat. Nos. 6,929,749 and 7,122,148 appear to disclose methods for inhibiting silica scale formation and corrosion in aqueous systems by pre-removal of hardness ions from the source water, maintenance of electrical conductivity, and elevation of the pH level. U.S. Pat. No. 4,995,986 appears to disclose a method of removing contaminants from wastewater by the addition of aqueous solutions of magnesium chloride and sodium silicate; however, it appears to focus on the in situ precipitation of an amorphous magnesium silicate by a controlled process of addition of magnesium salts and silicate salts—which could increase the amount of scale—and the subsequent removal of pollutants from the liquid media, which is not expected to give significant improvement in the removal of scalants. U.S. Pat. No. 4,713,177 may disclose a process for reducing calcium, magnesium, and aluminum salt scale build-up by adding a precipitating reagent to preferentially precipitate calcium, magnesium, and aluminum ions. PCT International Publication No. WO 84/02126 appears to disclose a method for preventing formation of calcium and magnesium scales by adding low or negligibly water soluble alkali metal silicates or silicic acid. However, those references do not appear to disclose preventing or reducing scale formation or corrosion by adsorbing to scale crystals a non-aqueous, particulate scale-adsorbent agent, such as calcium silicate.